Parasitism and hnmnnitv. 45 



CHAPTER VIII. 



Pakasitism and Immunity. 



In cGiinexiou witli tlio diseases of plants, where fungus parasites are con- 

 cerned, determined efforts have been made to study these parasites in such a 

 way that their Ufe-histories might be made out, and their mode of infecting 

 the host-plant and producing the disease thoroughly understood, the object 

 being, from the knowledge thus obtained, to prevent the parasite gaining an 

 entrance into the host. This has hitherto been usually accomplished, either 

 by destroying the spores of the fungus or preventing their germination, and 

 thus effectually preventing the appearance of the disease. By means of spray- 

 ing, for instance, at the proper time with suitable fungicides, a large number 

 of such diseases have been kept in check, and so successful had this method 

 proved that the hope was entertained by many that it only required a further 

 extension of our knowledge of plant parasites and their life histories, in order 

 to completely overcome their destructive effects. This was a great step in 

 advance, even although it involved unceasing warfare against the parasite, 

 and the Department of Agriculture of the United States of America led the 

 way in demonstrating the efficacy of the gospel of spraying. 



But there were some serious diseases due to parasitic fungi which did not 

 •lend themselves to this mode of treatment, even although the history of the 

 parasite had been followed from spore to spore, and it became necessary to 

 devise other means for controlling the disease. Foremost among these was 

 the Rust in Cereals, to which, from the very nature of the mode of attack of 

 the parasite, and the manner of growth of the host-plants themselves, 

 it was found to be mechanically impossible to apply this method of 

 prevention. 



It will be observed that in this method of grappling with disease, the host- 

 plant itself is left out of account, and attention is concentrated on the para- 

 site. But there is another possible way of preventing disease, and that is by 

 rendering the plant immune. Accordingly, attention was turned to the question 

 of immunity or resistance to disease, which has been for some time deepiy 

 studied and discussed in connexion with human diseases, and which has also 

 been observed in some species and varieties of cultivated plants, such as 

 wheats and other cereals. The definition of an immune plant as given by 

 Massee^ may be accepted. It signifies an individual of the same species as the 

 one on which a given species of fungus is parasitic, but which, owing to the 

 absence of the chemotactic substance in its tissues necessary to enable the 

 germ-tubes of the fungus to penetrate, remains unattacked. Only it is to be 

 understood that while the parasite may gain an entrance, it does not normally 

 develop in such an " immune " plant, and, consequently, cannot produce the 

 harmful disease or spread infection by means of its spores. 



In dealing with the smuts \\dth which we are more immediately concerned, 

 there were various steeps for the seeds, or disinfectants, as they may be called, 

 such as sulphate of copper or bluestone, and formalin, which effectually pre- 

 vented the attacks of some of the more common smuts, but there were othei's, 

 su.ch as the destructive flag smut of wheat, which did not yield to such treat- 

 ment, and even here it is becoming apparent that immunity to the disease is to 

 be aimed at as a means of overcoming it. In short, at the presen' time, it is 

 felt that our present methods of fighting parasitic fungi is simply tinkering 



